1. Field of the Invention
The present invention relates to a conveying device for conveying a work piece such as a silicon wafer, for example.
2. Description of the Related Art
For etching of a wafer, CVD (chemical vapor deposition) and the like, for example, it is necessary to convey the wafer in a multi-chamber in a clean and ultra-high vacuum environment. In such an environment, the conveying device operates. A conveying device which does not require the use of a magnetic fluid seal, that is, a conveying device provided with a separating wall between a rotor and a stator in a motor has been devised in order to prevent the environment in the chamber from being deteriorated. The conveying devices described in Japanese Patent No. 2,761,438 and U.S. Pat. No. 5,720,590 are examples of such conveying devices. FIG. 9 is a longitudinal sectional view showing a conveying device 101 having the same basic structure as the conveying devices described in the above-mentioned publications. The conveying device 101 comprises a coaxial shaft mechanism including a first shaft 121 and a second shaft 122 which are independently rotatable, and a conveying arm assembly 130 fixed to the upper ends of the shafts 121 and 122. The first shaft 121 extends downward from the lower end of the second shaft 122 and penetrates the second shaft 122. A rotor Rxe2x80x2 is attached to the outer peripheral sides of the first shaft 121 and the second shaft 122, and a stator Sxe2x80x2 is attached to a housing 190 accommodating the first shaft 121 and the second shaft 122. A motor Mxe2x80x2 is constituted by the rotor Rxe2x80x2 and the stator Sxe2x80x2. By controlling the rotation of the motor Mxe2x80x2, the expansion, contraction and turn of the conveying arm assembly 130 can be controlled. The reference numeral 145 denotes an optical encoder for detecting the rotation of the first and second shafts 121 and 122.
In the conveying device 101 having such a structure, it is required that the conveying arm assembly 130 should be quickly set in a conveying position and be rapidly stabilized in the conveying position. For this purpose, adequate characteristics are required for the shafts 121 and 122. FIG. 10 is a chart showing a process of controlling the rotation of the shaft, wherein an axis of ordinate indicates angular velocity of the shaft and an axis of abscissa indicates time. In general, the rotation of the shaft is controlled to reach a stopping step xe2x80x9cexe2x80x9d from a stopping step xe2x80x9caxe2x80x9d through an accelerating step xe2x80x9cbxe2x80x9d, a constant-velocity rotating step xe2x80x9ccxe2x80x9d and a decelerating step xe2x80x9cdxe2x80x9d as shown in FIG. 10. In the conveying device 101, it is necessary to rapidly accelerate or decelerate the shafts 121 and 122, that is, to increase an angular acceleration at the accelerating step xe2x80x9cbxe2x80x9d and an angular deceleration the decelerating step xe2x80x9cdxe2x80x9d shown in FIG. 10 in order to quickly set the conveying arm assembly 130 in the conveying position. Moreover, the oscillation of the angular velocity is observed in the early stage of the constant-velocity rotating step xe2x80x9ccxe2x80x9d and that of the stopping stage xe2x80x9cexe2x80x9d in FIG. 10. In order to quickly stabilize the conveying arm assembly 130 in the conveying position, however, it is necessary to reduce times t1 and t2 taken to cause the oscillated angular velocity to converge on a constant value, that is, stabilizing times. With an increase in the size of the work piece, furthermore, the conveying device should have the characteristics that a conveying distance is long and the conveying device is resistant to a great load. In order to satisfy these requirements, the torsional rigidity of each of the shafts 121 and 122 should be increased. If it is desired to increase the torsional rigidity of each of the shafts 121 and 122, it is necessary to shorten the shafts 121 and 122 or to increase a modulus of section of each of the shafts 121 and 122.
Moreover, when the conveying arm assembly 130 connected to the two shafts 121 and 122 is to be driven, the synchronous driving of the two shafts 121 and 122 is required. For this purpose, it is necessary to reduce a difference in the torsional rigidity between the two shafts 121 and 122. In order to reduce the difference in the torsional rigidity between the two shafts 121 and 122, it is necessary to reduce a difference in a length between the two shafts 121 and 122 and a difference in a modulus of section between the shafts 121 and 122.
In the conveying device 101, however, the first shaft 121 extends downward from the lower end of the second shaft 122 to penetrate the second shaft 122. For this reason, particularly, it is hard to reduce the length of the inside shaft 121 and to increase an outside diameter thereof. If the outside diameter is increased, the inside and outside diameters of the second shaft 122 should also be increased. Consequently, the outside dimensions and weights of both the shafts 121 and 122 are increased. Therefore, a large-sized motor is required for controlling the expansion, contraction and turn of the conveying arm assembly 130. Moreover, it is impossible to avoid an increase in the outside diameter of the housing 190.
With the structure of the conveying device 101, furthermore, the shaft 121 has a greater length and a smaller modulus of section than the shaft 122. Therefore, the difference in the torsional rigidity between both the shafts 121 and 122 is great. Accordingly, both the shafts 121 and 122 cannot be synchronously driven by rapid acceleration and deceleration.
In order to perform positioning with high precision, run-out of the shaft should be small. In the conveying device 101, the run-out is generated on the shaft 122 due to the precision of a bearing 100B during the rotation thereof. Similarly, when the shaft 121 is relatively rotated with respect to the shaft 122, relative run-out is generated on the shaft 121 with respect to the shaft 122 due to the precision of a bearing 100Bxe2x80x2. In the conveying device 101, therefore, when the shaft 121 and the shaft 122 are rotated at the same time, accumulative run-out is generated on the shaft 121 due to the precision of each of the bearings 100B and 100Bxe2x80x2. Consequently, it is impossible to perform the positioning with high precision.
The conveying device 101 has such a structure that the operation of one conveying arm assembly 130 is controlled by a set of shafts 121 and 122. If the operations of a plurality of conveying arm assemblies are to be controlled by plural sets of shafts, the above-mentioned problems become more remarkable. For example, if two conveying arm assemblies are to be controlled by two sets of shafts, four shafts are coaxially provided to control the operation of one of the conveying arm assemblies by means of two inner shafts and that of the other conveying arm assembly by means of two outer shafts. With such a structure, it is harder to reduce the length of the inner shaft and to increase the modulus of section thereof. Furthermore, the torsional rigidity of the inner shaft cannot be increased. Moreover, the lengths and moduli of section of innermost and outermost shafts have very great differences. Therefore, a difference in the torsional rigidity becomes very great. In particular, the innermost shaft is attached to the housing through much more bearings. Therefore, the accumulation of the run-out due to the precision of the bearing is increased so that the run-out becomes very great, resulting in poor positioning precision.
It is an object of the present invention to provide a conveying device in which the torsional rigidity of a shaft can be enhanced without increasing the dimension of the conveying device and a difference in the torsional rigidity between a plurality of shafts can be reduced, and furthermore, run-out due to the precision of a bearing is not accumulated.
The present invention provides a conveying device comprising a conveying arm assembly, a fixed shaft, at least one set of hollow operating shafts which are necessary for controlling an operation of the conveying arm assembly, and a motor provided between the fixed shaft and each of the operating shafts. The one set of operating shafts are attached to the fixed shaft such that they can be rotated coaxially with respect to the fixed shaft on an outside of the fixed shaft and are arranged in an axial direction of the fixed shaft. The motor comprises a stator provided on the fixed shaft and a rotor provided on each of the operating shafts such that it is opposed to the stator on an outside of the stator. Thus, the operating shafts are provided on the outside of the fixed shaft. Consequently, the outside diameters and moduli of section of the operating shafts are increased. Even if the outside diameters are increased, the weights are comparatively small because the operating shafts have hollow structures and annular cross-sections. Moreover, it is not necessary for one of the operating shafts to penetrate the other operating shaft. Therefore, both the operating shafts can be shortened. Thus, the torsional rigidity of the operating shaft can be enhanced without increasing the dimension of the conveying device.
In addition, the lengths and cross-sectional shapes of both the operating shafts can be set almost identically. A difference in the torsional rigidity between both the operating shafts can be reduced.
Thus, the torsional rigidity of the operating shaft can be enhanced without increasing the dimension of the conveying device. Therefore, the operating shaft can be rapidly accelerated and decelerated. In addition, synchronous driving can be performed at a high speed by reducing a difference in the torsional rigidity between a plurality of operating shafts. Consequently, the conveying arm assembly can be quickly set in the conveying position. Furthermore, even if the angular velocity of the operating shaft is oscillated, it rapidly converges. Therefore, it is possible to reduce a time taken for stabilizing the conveying arm assembly in the conveying position. As a result, a work for setting the conveying arm assembly in the conveying position can be rapidly performed.
Moreover, a bearing can generally be used for rotatably holding the operating shafts. Each of the operating shafts are not held through a bearing attached to the other operating shaft but are directly held by means of a bearing attached to the fixed shaft. Therefore, run-out can be reduced without the accumulation of the run-out of the operating shaft.
Furthermore, the rotor which serves as a point of action of the motor is situated in a point which is more distant from the center of rotation. Therefore, it is possible to obtain a necessary torque even if the height of the rotor is reduced. Accordingly, the torsional rigidity can be increased by shortening the operating shaft. In addition, the height of the conveying device can also be reduced.
The torsional rigidity of the operating shaft is increased. Consequently, a resonance frequency can be more increased than a frequency included in a motor driving signal. Thus, a resonance can be avoided.
The above-mentioned conveying device may further comprise a plurality of conveying arm assemblies and plural sets of operating shafts for controlling operations of the conveying arm assemblies. Even if the number of the operating shafts is increased by such a structure, the outside diameters of the operating shafts do not need to be reduced and their torsional rigidities can be kept great. Moreover, even if the number of the operating shafts is increased, the cross-sectional shapes of all the operating shafts can be set almost identically. Consequently, a difference in the torsional rigidity can be reduced. Furthermore, even if the number of the operating shafts is increased, respective bearings can be all attached to the fixed shaft. Therefore, the run-outs of all the operating shafts can be reduced almost identically.
In the above-mentioned conveying device, furthermore, a rotation detecting portion capable of detecting rotation of the operating shaft may be provided between the fixed shaft and the operating shaft. The rotation detecting portion may be constituted by a resolver type position detector or an optical encoder, for example.
In the above-mentioned conveying device, moreover, the fixed shaft may be attached, with airtightness, to a wall portion of a vacuum chamber so that the conveying arm assembly can be put in a vacuum environment.
Furthermore, the above-mentioned conveying device may comprise a lift mechanism for bringing the fixed shaft up and down in order to bring the conveying arm assembly up and down. In this case, the fixed shaft may be attached to a wall portion of a vacuum chamber through the lift mechanism and a flexible seal member may be provided between the fixed shaft and the wall portion of the vacuum chamber so that the conveying arm assembly can be put in a vacuum environment.
In the above-mentioned conveying device, moreover, a stator accommodating space isolated from an outer peripheral face of the fixed shaft may be formed in the fixed shaft, the stator being accommodated in the stator accommodating space. According to such a structure, the stator can be put in the space isolated from the space where the operating shafts are present. Consequently, in particular, also in the case where the stator is attached to the vacuum chamber, the vacuum environment is not deteriorated.
A concave portion may be formed on the fixed shaft to be opened on the outer peripheral face of the fixed shaft and the opening of the concave portion may be closed with airtightness by a separating wall member so that the stator accommodating space is formed.
In the above-mentioned conveying device, furthermore, a passage for communicating from an end face of the fixed shaft to the stator accommodating space may be formed in the fixed shaft. According to such a structure, heat generated on the stator can be discharged to an outside space through the passage. Moreover, the passage can also be utilized for distributing an electric wire to supply power to the stator.
The object as well as other objects, features and advantages of the invention will become more apparent to those skilled in the art from the following description with reference to the accompanying drawings.